Many diverse methods of optical imaging and assisted viewing require some type of specimen illumination. Microscopes magnify light from only a small area with a concomitant reduction in light intensity. This necessitates intense illumination of the specimen in order to provide adequate light levels for viewing. Machine vision applications often achieve better performance with intense illumination, which provides better resolution of the workpiece's or specimen's details. Identifying workpiece edges and surface features allows the position and orientation to be determined automatically.
Generally the lighting can originate from in front of or behind the specimen. Microscopes commonly use backlighting with a light source below a specimen stage that directs light upward through a microscope slide held on the stage to its first optical element, or the microscope objective. This lighting configuration is typically used for a transparent or translucent specimen since the light must pass through the specimen to be captured by the microscope. Light sourced from above the specimen is typically used with opaque specimens.
When lighting from above the specimen, a problem stems from the fact that a single point light source can not be located in line with the microscope's optical axis without obstructing its view or a complex optical arrangement. The specimen is usually illuminated from light off the axis which can create shadows. The solution to this dilemma is a device termed a ringlight. This device is typically torroidal in shape to fit around the barrel of the microscope or similar optical apparatus. The barrel and ringlight are arranged so that the barrel is coextensive with the ringlight's axis. The ringlight emits light in a 360 degree circle in the general direction of its axis, but angling inward slightly. This forms a cone of light having a vertex located on the axis. When used as a microscope light, for example, the specimen is illuminated evenly from all sides without shadows being visible through the microscope.
There are a number of variations on the backlighting theme. One such variation is a form of differential illumination termed dark field illumination. Many specimens exhibit little or no contrast when viewed with ordinary backlighting because they are colorless and transparent. Chemical staining is the typical solution in this situation, but in some cases it may be undesirable. In dark field illumination, the specimen is illuminated with a hollow cone of light aligned along the optical axis of the microscope and originating from below the specimen. The microscope objective is located within the dark base of the hollow light cone. Consequently, without a specimen, there is no illumination in the microscope. A specimen placed on the stage, however, tends to diffract, reflect, and refract light of the cone, and this scattered light can then enter the objective. When observed through the microscope, live bacteria, for example, are visible, their edges and internal structures being outlined by redirected light, which is collected by the microscope optics. Rheinberg differential illumination is similar except that the field is given a desired color with diffuse lighting.